FIELD OF THE INVENTION
The invention relates to a circuit for an NOx measurement sensor.
In order to measure the NOx concentration in a gas, for example in the exhaust gas of an internal combustion engine, it is known to employ a thick-layer measurement sensor. Such a measurement sensor is described, for example, in the publication by N. Kato et al., titled "Thick Film ZrO.sub.2 NOx Sensor for the Measurement of Low NOx Concentration", Society of Automotive Engineers, Publication Number 980170, 1989, or in the publication by N. Kato et al., titled "Performance of Thick Film NOx Sensor on Diesel and Gasoline Engines", Society of Automotive Engineers, Publication Number 970858, 1997. The measurement sensor has two measurement cells and is composed of a zirconium oxide that conducts oxygen ions. It implements the following measurement concept: in a first measurement cell, to which the gas to be measured is fed via a diffusion barrier, a first oxygen concentration is established by use of a first oxygen ion pump current, the intention being that no decomposition of NOx takes place. In a second measurement cell, which is connected to the first via a diffusion barrier, the oxygen content is lowered further by use of a second oxygen ion pump current. The decomposition of NOx at a measurement electrode leads to a third oxygen ion pump current, which is a measure of the NOx concentration. The entire measurement sensor is in this case brought to an elevated temperature, for example 750.degree. C., by an electric heater.
In order to measure the third pump current, representing the measure of the NOx concentration, the current is normally fed through a measurement resistor, and the voltage drop across the resistor is tapped off and measured. In order to achieve the required accuracy, this measurement has to be carried out with a relatively high resolution. If it were wished to use a microcontroller that is normally employed for such measurement tasks, it has been shown that an analog/digital converter with a resolution of 8 bits is not adequate for this. The difficulties of current measurement are therefore to be seen essentially in the necessary resolution of the analog/digital converter and in measurement errors in the converter (offset, gain, etc.). It is therefore necessary to employ more expensive microcontrollers, of which only a few models are available on the market, so that the choice for the production or configuration is restricted here.